The present invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to a method and system for parallel imaging.
Magnetic Resonance Imaging (MRI) is a well-known medical procedure for obtaining detailed, one, two and three-dimensional images of patients, using the methodology of nuclear magnetic resonance (NMR). MRI is well suited to the visualization of soft tissues and is primarily used for diagnosing disease pathologies and internal injuries.
Typical MRI systems include a superconducting magnet capable of producing a strong, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including transmitter and receiver coils, also surrounding or impinging upon a portion of the patient; a magnetic gradient coil system also surrounding a portion of the patient; and a computer processing/imaging system, receiving the signals from the receiver coil and processing the signals into interpretable data, such as visual images.
The superconducting magnet is used in conjunction with a magnetic gradient coil assembly, which is temporally pulsed to generate a sequence of controlled gradients in the main magnetic field during a MRI data gathering sequence. Inasmuch as the main superconducting magnet produces a homogeneous field, no spatial property varies from location to location within the space bathed by such field; therefore, no spatial information, particularly pertaining to an image, can be extracted therefrom, save by the introduction of ancillary means for causing spatial (and temporal) variations in the field strength. This function is fulfilled by the above-mentioned gradient coil assembly; and it is by this means of manipulating the gradient fields that spatial information is typically encoded.
The actual image data consist of radiofrequency signals, which are stimulated and received by means of systems of resonant radiofrequency coils which irradiate the patient in the scanning volume. These coils typically fall into two classes: volume and surface resonators.
A volume resonator encloses an entire volume to be irradiated; a surface resonator lies atop a surface portion of some object to be irradiated. A volume resonator is typically employed as a combined transmitter/receiver, or as a dedicated transmitter, with no receiver functionality. A surface coil is sometimes employed as a combined transmitter/receiver; but more frequently it is used as a dedicated receiver, in conjunction with a separate volume coil serving as a transmitter. The surface receiver coil must be fitted with switchable blocking circuitry, to quench its response to the externally applied radiofrequency fields during the transmit period. Multiple surface resonators may, if suitably fitted with switchable protection circuitry, be further combined into receiver arrays, (for use with a separate transmitter coil) as is known in the art and described in more detail hereinbelow. Furthermore, since individual surface resonators (belonging to a receiver array) possess each a degree of spatial discrimination, due to the spatial variation of their reception sensitivity, and particularly to its localization in regions of close proximity to each surface coil, it is therefore possible to derive therefrom a degree of spatial information, based upon said localization of sensitivity by individual coils, whose sensitivity variations have been previously characterized in a suitable calibration sequence. Having available such information renders redundant a portion of the spatial encoding derived from the temporal sequencing of gradient fields, so that certain portions of the gradient sequence can be dispensed with, and, by suitable reconstruction means as known in the art, an image be reconstructed with limited gradient data, combined with localizing information derived from the radiofrequency coils. Since a portion of the gradient sequence is rendered redundant, the data collection process is shortened thereby, and the imaging process speeded. This essentially is the method of parallel imaging. It is to be noted that the larger the number of surface coils producing local information, the larger the acceleration of data gathering that can be realized.
To elaborate on the above: in the course of a typical MRI scan, RF signals of short duration (pulses) at suitable frequencies and in suitable temporal sequence are transmitted into the scanning bore by means of RF coils mentioned above. These pulses stimulate in the patient (or object to be imaged) nuclear magnetic resonance (NMR) signals in response, which are then allowed to evolve under then influence the abovementioned gradient sequences, and are (at times concurrent with playing of a gradient) received via suitable RF coils. Information encoded within the frequency and phase parameters of the received RF signals is then processed (by a suitable application of Fourier transform techniques, as known in the art) to form visual images. These visual images represent the distribution of an NMR sensitive nucleus (typically hydrogen), as weighted by relaxation factors, within a cross-section or volume of the patient within the scanning bore.
The present invention, as will be discussed in the specification, is particularly concerned with a particular type of radiofrequency resonator, known in the terminology of MRI as a transverse electromagnetic wave resonator (hereinafter TEM resonator or TEM), which is a type of multimode resonator, used to preferentially excite a particular electromagnetic frequency. Typically the resonator comprises an outer conductive cylinder containing within, as will be described, a plurality low-pass pi resonant elements of rod-like aspect, whose axes are aligned parallel to that of the outer cylinder. The low pass elements are grounded to the cylinder, which therefore serves the dual functions of electromagnetic shield and current return path.
In many typical TEM resonators, the pi circuits are fabricated from segments of coaxial line, by making a gap in the center conductors at their midpoints. The length of the gap determines the low pass capacitance; the overall length and diameter of the segment, together with its stand off distance from the shield together determine the low-pass inductance.
In other typical TEM resonators, the segments are simply self-resonant, axially-directed rods, with a very small lumped capacitance at either end to provide electrical foreshortening. By extension, the axial rods may be chosen far from self-resonance, and a large capacitance applied to achieve resonance. Since the shield still provides the return path, the resonator is referred to as the lumped element TEM.
It is known that the TEM configuration is adaptable to surface coil geometry. The principal advantage to this is geometric simplicity and the partial shielding achieved, particularly for imaging fields above 3.0 T. TEM geometry has also been adapted to multi-element surface arrays, but designs to date have been constrained to quarter wave sections (distributed or lumped) for the elements, to achieve (what has until now been considered) adequate inter-element circuit decoupling, or isolation: hitherto considered necessary to prevent image artifacts arising from inter-channel cross-talk among individual elements of the array. In conventional receiver arrays (typically comprising loop resonators) the isolation is achieved by a combination of overlapping the loops, plus the use of a low-impedance preamplifier in conjunction with the circuitry already in place to block the transmitter pulse (vide supra).
The particular requirement for quarter wave elements in TEM arrays is due to the rather strong coupling which exists between the elements and which is not altogether overcome by the blocking circuitry and preamplifier. This requirement restricts the resonant element geometry in ways that are potentially undesirable, e.g. to achieve quarter wave lumped elements, length and conductor spacing must combine to produce an inductive reactance of 50 ohms.
Now it would be desirable to have a TEM resonator without requiring quarter wave sections, to allow MRI designers to tailor resonator geometry for optimal coverage of the patient to be imaged. Abandoning the constraint of quarter wave elements still leaves the necessity of applying a certain degree of isolation from the otherwise strongly coupled TEM resonant elements. The requisite isolation is deemed achievable via the conventional isolation circuitry and low impedance preamplifier, as is known in the art. In this regard it has recently been shown theoretically (13) that parallel imaging reconstruction is highly resistant to formation of artifacts from inter-channel cross-talk among array elements; and therefore, surface receiver arrays intended exclusively for parallel imaging are to some degree exempt from the usual rigorous requirements of isolation between channels; an inter-channel isolation of 10 dB is now considered tolerable between next-neighbor elements, whereas in the conventional imaging regime, the tolerable isolation was considered to be in the region of 20 to 30 dB.